The invention relates to a flexible shaft for transmitting rotary motion, to a rotary motion transmission device including such a flexible shaft, and to an adjustment system for adjusting a motor vehicle seat, which system includes such a device.
Transmission devices are known that comprise a flexible shaft and a sheath inside which the shaft is received. In particular, the flexible shaft can be formed of one or more core wires made of steel and around which winding wires are wound, the winding wires also being made of steel. In such a transmission device, in particular for adjusting a motor vehicle seat, the speeds of rotation of the flexible shaft in the sheath are higher than 2000 revolutions per minute (r.p.m.), and conventionally about 3000 r.p.m. During rotation at such speeds, vibration of small amplitude appears that propagates along the flexible shaft and that give rise to noise. Trials conducted by the Applicant have shown that one of the causes of the appearance of such vibration is that the elasticity and resilience of the steel core wires used to make the flexible shaft are too high. An object of the invention is to mitigate that problem by proposing a flexible shaft whose core wires present elasticity and resilience that are lower while preserving good strength.
The Applicant has observed that the function of the core wire is not to transmit the rotary torque, but rather to serve as a support or strength member for the winding wires. Thus, it is possible to reduce the elasticity and resilience of the core wire while preserving the performance of the flexible shaft. Such performance is preserved by means of the good strength of the core wire. Said strength makes it possible to provide a good support for the steel winding wires that transmit the rotary torque. Thus, the combination of a core wire having low elasticity and of winding wires that are elastic makes it possible to obtain a reduction in the vibration phenomenon and also to preserve the performance of the flexible shaft.
To this end, and in a first aspect, the invention provides a flexible shaft designed to rotate inside a sheath for the purpose of transmitting rotary motion, said shaft comprising at least one core wire and winding wires wound around said core wire, the core wire presenting elasticity and resilience that are lower than the elasticity and resilience of the winding wires. In a second aspect, the invention provides a rotary motion transmission device comprising a sheath and a flexible shaft received inside the sheath, the sheath comprising a core formed of a hollow cylinder whose inside diameter is arranged to enable said shaft to rotate inside said sheath, the flexible shaft being a shaft as described above.
In a third aspect, the invention provides an adjustment system for adjusting a motor vehicle seat, said system comprising at least one adjustment runner mounted on the structure of the vehicle and adjustable means for fastening the seat to said runner(s), said system further comprising a drive motor provided with at least one rotary outlet, said adjustment system further comprising a transmission device as described above and that is disposed between the outlet of said drive motor and said fastening means, so as to move said fastening means along said adjustment runner in response to said outlet rotating.